The present invention relates to a rare earth-based permanent magnet or, more particularly, to a permanent magnet which is a sintered body of a rare earth-based alloy having excellent magnetic properties prepared by a powder metallurgical process and useful as a component of various kinds of electric and electronic instruments as well as a method for the preparation of the rare earth-based permanent magnet.
Among the various types of rare earth-based permanent magnets hitherto developed and currently used in many applications, a recently highlighted class of the magnets includes those having an alloy composition of neodymium, iron and boron as the essential alloying elements. These neodymium-iron-boron magnets have very excellent magnetic properties equivalent to or even better than the previously developed samarium-cobalt magnets and are still advantageous in respect of the abundance of the neodymium resources in comparison with samarium contained in rare earth minerals only in a relatively minor content as well as the inexpensiveness of iron in comparison with cobalt (see, for example, Japanese Patent Kokai 59-46008).
Despite the generally excellent magnetic properties, the neodymium-iron-boron magnets are not free from a problem because the Curie point T.sub.c of the magnets is relatively low, for example, at 312.degree. C. or below for the phase of an intermetallic compound of Nd.sub.2 Fe.sub.14 B. Consequently, the temperature dependency of the magnetic properties is large to cause limitations in the use of these permanent magnets at elevated temperatures. In particular, the coercive force .sub.i H.sub.c greatly decreases by the increase in temperature to such an extent that the magnets cannot be used as such in many applications. An attempt has been made in this regard to increase the coercive force of the magnet at room temperature by the admixture of a certain additive to the neodymium-iron-boron alloy to such an extent that the coercive force even after decrease by a possible temperature increase during use may still be high enough not to lose the practical usefulness of the magnet. The hitherto proposed additives for such a purpose include, for example, so-called heavy rare earth elements such as dysprosium, terbium, holmium and the like, transition metals such as titanium, vanadium, niobium, molybdenum and the like and aluminum (see Japanese Patent Kokai 59-898401 and 60-32306).
Although these additive elements indeed have an effect to increase the coercive force of the neodymium-iron-boron magnets, the residual magnetic flux B.sub.r of the magnets is necessarily decreased by the addition of these additives. Therefore, it is an important problem that the coercive force of the magnet can be sufficiently increased with a minimum decrease in the residual magnetic flux by appropriately selecting the kinds and combination of the additive elements. In particular, the heavy rare earth elements have a larger effect of increasing the coercive force than the other additive elements but at a sacrifice of a large decrease in the residual magnetic flux as a consequence of the anti-parallel alignment of the magnetic moments in the heavy rare earth element and iron. In addition, these heavy rare earth elements are contained in the rare earth minerals only in very low contents so that they are necessarily very expensive and the amount of addition of these heavy rare earth elements in the magnet alloys should be as small as possible also for the economical reason.